The present invention relates to a key telephone system, and more specifically to a key telephone system provided with digital key telephone sets connectable to add-on modules for enhancement of telephone functions.
FIG. 1 is a block diagram showing the entire system configuration of a prior-art key telephone system. The key telephone system comprises a key service unit (referred to as KSU, hereinafter) 1, and a plurality of digital key telephone sets (referred to as DKTs, hereinafter) 10 and 20, to which add-on modules (referred to as ADMs, hereinafter) are optionally connectable to enhance the functions of the DKTs.
In FIG. 1, the KSU 1 comprises an office line interface unit 2 connected to an office line 17a, 17b, and a plurality of DKT interface units 4 and 5 (only two units are shown) provided with a plurality of ports to each of which each of the DKTs is connected. Further, the KSU 1 includes a time switch 3, and a central control unit 6 for controlling the entire system. The central control unit 6 has a CPU 7, a ROM 8, and a RAM 9.
On the other hand, the DKT 10 comprises a data transmitter 11, a speech transmitting/receiving circuit system 12, and a CPU 13, with further dial keys, extension telephone set selection keys, office line selection keys, etc. (all not shown).
The ADM 14 comprises a selector circuit 15, a CPU 16, normally-open contacts 35a to 35n, and a keyboard (which are all not shown). The selector circuit 15 includes AND gates 15a and 15c, an inverter circuit 15b, and an OR gate 15d. The normally-open contacts 35a to 35n are closed, respectively, when the one-touch keys arranged on the keyboard of the ADM 14, so as correspond to these normally-open contacts 35a to 35n, are depressed, respectively. These one-touch keys access read compacted telephone dial data previously stored in the RAM 9 of the central control unit 6 of the KSU 1, for instance.
The configuration of the DKT 10 and the ADM 14 will be described hereinbelow in further detail.
The data transmitter 11 receives or transmits a packet including speech data (PCM codes) from or to the DKT interface unit 4 of the KSU 1, in accordance with the ping-pong transmission method, for instance. The data transmitter 11 outputs the speech data in the received packet to the speech transmitting/receiving circuit system 12, and the control data in the received packet to the CPU 13 and the ADM 14. Further, the data transmitter 11 receives the speech data outputted from the speech transmitting/receiving circuit system 12 and the control data outputted from the CPU 13 and the ADM 14, and transmits the packet including these data to the DKT interface unit 4.
The speech transmitting/receiving circuit system 12 includes a coded, a speech amplifier, a handset, etc., which are all not shown.
The CPU 13 receives control data from the data transmitter 11, and checks whether a data output request signal is included in the received control data. Here, the up-data output request signal is given periodically from the central control unit 6 in the KSU 1 to the DKT 10 and the ADM 14, to request the DKT 10 and the ADM 14 to output some control data. When some control data must be transmitted to the KSU 1, the CPU 13 detects the data output request signal from the control data received from the data transmitter 11, and gives a logical "H" level data transmit disable signal to one input terminal of the AND gate 15a in the selector circuit 15 of the ADM 14, an input terminal of the inverter circuit 15b, and the CPU 16. Further, the CPU 13 outputs control data in series to the other input terminal of the AND gate 15a, so that control data outputted from the CPU 13 are inputted to the data transmitter 11. On the other hand, if the CPU 13 detects the data output request signal in the control data transmitted from the data transmitter 11, in spite of the fact that it is unnecessary to transmit the control data to the KSU 1, the CPU 13 gives a logical "L" level data transmit enable signal to the other input terminal of the AND gate 15a, an input terminal of the inverter circuit 15b, and the CPU 16. Under these conditions, if any one of the normally-open contacts 35a to 35n of the ADM 14 is closed, the CPU 16 outputs the control data corresponding to the closed contact to the AND gate 15c in series, so that the control data are inputted to the data transmitter 11.
As described above, when the data output request signal is given from the KSU 1, the control data from the CPU 13 of the DKT 10 or from the CPU 16 of the ADM 14 are selectively transmitted to the KSU 1. Further, when no ADM 14 is connected to the DKT 10, the data transmitter 11 and the CPU 13 are directly connected to each other so that various control data outputted from the CPU 13 can be directly given to the data transmitter 11.
In the above-mentioned prior-art ADM 14, however, since the control data are transmitted to the KSU 1 from the CPU 13 of the DKT 10 and from the CPU 16 of the ADM 14 by selectively adjusting the selector circuit 15 composed of two AND gates, an inverter circuit, and an OR gate, there exists a problem in that the number of parts in the ADM 14 (or ADM 18) inevitably increases, and the cost thereof increases. In addition, it has been impossible to further connect another ADM other than the ADM 14 to the DKT 10, in order to further increase the functions of the DKT 10.
To overcome the above-mentioned problem, a method has been proposed such that a plurality of ADMs are connected to a plurality of ports of the DKT interface unit 4, respectively. In the DKT which adopts this method, since the DKT 10 and the plural ADMs are connected in parallel to the KSU 1, it is possible to directly control the control data transmitting timing by the central control unit 6, so that the control data outputted from the DKT 10 and plural ADMs are not superposed upon each other, without need for the above-mentioned selector circuit 15.
In the above-mentioned method, however, since the ADMs are directly connected to the ports of the DKT interface unit 4, the ports corresponding to the number of the connected ADMs are used or occupied, with the result that the number of connectable DKTs is inevitably reduced.